In situ Chemical Analysis of Rising Bubbles Along the Cascadia Margin

Ebullition of methane from seafloor hydrocarbon seeps represents an under-constrained transfer mechanism as bubbles travel upwards through the water column. While bubble-hosted gases are known to diffuse into the surrounding water column, very little empirical evidence has been gathered to help evaluate models of such gas transfer - limiting efforts to constrain seafloor methane fluxes into the overlying ocean. Here we present the design and deployment of a novel bubble sampling system serving as a front end for in situ measurements of bubble chemical composition via an in situ mass spectrometer and carbon isotopes of methane via an in situ laser spectrometer. The bubble analysis system was deployed in 2018 using the ROV Subastian on the R/V Falkor at sites along the Cascadia Margin - a region well known for its active methane bubble flares. Data were collected during dives to seep sites at McArthur Ridge, Hydrate Ridge and Heceta Bank. With this experimental system, a total of 22 bubble samples were collected and analyzed in situ. Bubble samples contained between 84.6 to 100% methane, with notable variability both spatially and temporally. Bubbles rising from a deep site at Heceta Bank exhibited anomalously elevated levels of carbon dioxide relative to other sites. By advancing technologies for in situ capture, transfer and analysis, our approach helps shed light on the fate of seafloor methane release. Here, we will present design elements of the new bubble capture and transfer system, as well as summarize results from the in situ chemical characterization of rising bubbles from these sites.